The present invention relates to gradient index optical fibers and, more particularly, to a gradient index lens array which transmits an image of an object at an object plane onto an image plane with minimum exposure modulation.
Image transmitters comprising bundled gradient index optical fibers are known in the art. U.S. Pat. No. 3,658,407 describes a light conducting fiber made of glass or synthetic resin which has a refractive index distribution in a cross section thereof that varies parabolically outward from a center portion thereof. Each fiber acts as a focusing lens to transmit part of an image of an object placed near one end thereof and an assembly of fibers, in a one or two-row array, transmit and focus a complete image of the object. The fiber lenses are produced under the tradename "SELFOC", the mark is registered in Japan and owned by Nippon Sheet Glass Co., Ltd.
These gradient index lens arrays have found use in a number of technologies; e.g. in construction of printed type optical circuits as disclosed in U.S. Pat. No. 3,922,062 and as a replacement for conventional optical systems in copiers as disclosed in U.S. Pat. Nos. 3,947,106 and 3,977,777.
Imaging systems which utilize gradient index lens arrays are inherently subject to a problem of non-uniformity of exposure of an image at an imaging plane. In the typical staggered two-row prior art device, overlapping subimages of individual fibers result in exposure non-uniformity in the direction perpendicular to the process direction. This non-uniformity can be minimized by greatly increasing the field overlaps but there is a tradeoff in reduced optical tolerances associated with this technique. Another approach to minimize this type of exposure non-uniformity was disclosed in an article by James D. Rees and William L. Lama "Some Radiometric Properties of Gradient Index Fiber Lenses", published on Apr. 1, 1980 in Applied Optics, Vol. 19, No. 7. Essentially, the article disclosed a method of minimizing spatial non-uniformities by optimum selection of the fiber parameters, length, gradient index constant and packing factor. This technique, however, may not lend itself to applications where, for example, system requirements call for a fiber length different from the optimum length.